1. Field of the Invention
The present invention relates to the field of removing particles from a surface and, more particularly, to the cleaning of semiconductor wafers by the removal of particulate contaminants from a wafer surface.
2. Prior Art
In the process of manufacturing devices on a semiconductor wafer, a number of steps require the cleaning of the surface of the wafer to remove unwanted particles. Such particles will generally contaminate subsequent processing steps and/or introduce defects which can result in the failure of the integrated circuit device being manufactured. Furthermore, such defects can result in lower yields, which ultimately impact the economic cost associated with the manufacturing of integrated circuit devices. Additionally, as the various circuit dimensions shrink to a submicron level, contamination constraints are made ever more stringent. For example, smaller size particles which may not have caused a defect on a 1.0 micron electrical line of a wafer will have a likelier chance of causing a defect on a 0.35 micron, 0.25 micron or 0.15 micron electrical line.
A variety of techniques have been proposed and implemented in the prior art in order to clean semiconductor wafers. Two of the more popular techniques are the use of ultrasonic and megasonic cleaning systems. Typically, 25 to 50 wafers are immersed in an alkaline solution of ammonium hydroxide, water and hydrogen peroxide for approximately 10 minutes. In the bath, the wafers are subjected to high frequency sound waves in the range of 25-1000KHz. Subsequently, the wafers are rinsed and dried, requiring additional time of 5-15 minutes. Ultrasonic systems use the lower frequency range while megasonic systems use the higher frequency range. A listing of various known techniques is well described in U.S. Pat. No. 4,817,652 (Liu et al.).
It should be emphasized that many of the prior art techniques, which may be effective in removing contaminating particles, require the use of chemicals which are harmful to the environment or to humans who must work with the chemicals. Waste disposal of such harmful chemicals adds another significant concern in protecting the environment. In order to be more sensitive to environmental issues, the semiconductor industry is concerned with developing new wafer cleaning techniques which are more "environment friendly."
For example, two recent techniques for cleaning wafers can be found in U.S. Pat. No. 5,062,898 (McDermott et al.) and in an article entitled "Ice Scrubber Cleaning", Ohmori et al., Technical Proceedings Semicon/Kansai-Kyoto, pp. 142-149 (June 21-23, 1990). Both of these techniques provide for a more "environment friendly" approach to wafer cleaning. McDermott et al. discloses a surface cleaning method using an argon cryogenic aerosol. Cleaning of contaminated surfaces is accomplished through a process of colliding solid (frozen) argon particles at high velocity against the surface to be cleaned. The Ohmori et al. article discloses the use of impacting solid ice particles to scrub the wafer surface.
However, both techniques (McDermott et al. and Ohmori et al.) still require the cooling of a gas or a liquid by the use of a cryogenic element. In McDermott et al., argon gas is cooled and solidified in an heat exchanger, which is cooled by liquid nitrogen. In Ohmori et al., nitrogen is used as a coolant to form ice particles from ultra pure de-ionized (DI) water and a carrier gas (nitrogen) is utilized to jet the ice particles.
The present invention provides for an "environment friendly" technique of cleaning a wafer, but without the necessary requirement of forming solid particles to blast or scrub the surface of the wafer.